This application pertains to the art of coating and, more particularly, to air spraying of coating materials.
The invention is particularly applicable for use in spraying a coating material on the inside surfaces of longitudinal seams on can bodies and will be described with particular reference thereto. However, it will be appreciated that the invention has broader aspects and may be used for air spraying coating materials on the outside surfaces of can body seams or surfaces other than can body seams.
In the manufacture of three-piece cans, it is common to provide a protective metal lining on one surface of a metal strip which is subsequently slit into rectangular blanks. The protective metal lining is also provided with an initial protective coating of suitable material including vinyls, epoxies or phenolics. It is possible to apply this initial protective coating either before or after the metal strip is slit into rectangular blanks. The metal blanks are formed into cylinders with the inside of the cylinders having the protective lining and coating. Opposite side edges of the blanks are secured together by welding, soldering or the use of adhesive to define longitudinal seams. The initial protective coating is not applied to the opposite side edge portions of the blanks which are to be overlapped to form the longitudinal seams. The longitudinal seams form a most vulnerable area where failures are likely to occur due to such imperfections as pinholes and cracks. Therefore, the interior surfaces of the longitudinal seams are commonly provided with a stripe of protective coating material of any suitable type, including vinyls, epoxies and phenolics.
The interior surfaces of the seams are coated with the stripe of protective coating material while the can bodies are moving longitudinally at a high velocity. Therefore, it is necessary that the spraying apparatus for applying the stripe of coating material to the seam area operate in and efficient and reliable manner. Overspraying or splattering results in having the stripe coating material located in undesirable areas beyond the longitudinal seam. The stripe coating material is commonly cured by inductively heating the can body in the area of the seam and the oversprayed or splattered material may remain uncured. In situations where a final protective coating is applied to the entire interior surface of the can body, to cover both the initial coating and the stripe coating, the oversprayed or splattered stripe coating material may not be cured at the relatively lower curing temperatures of the final coating material and this will cause imperfections in the finished coating. Overspraying and splattering can also result in blisters or cracks in the final coating. Application to the seams of a stripe coating which is too thick frequently results in blistering of the stripe coating material or any final coating material which may be applied.
Previous arrangements for applying the impervious stripe coating material to the interior surfaces of the longitudinal seams have included airless-type of spraying devices. Airless sprayers require relatively high pressures to produce a highly dispersed spray and the high pressures make the spray very susceptible to splattering. Increasing the viscosity of the coating material or lowering the pressure can result in a stripe protective coating which is too thick so that blistering occurs. In previous arrangements using air-type spraying devices, the nozzles have been positioned interiorly of the can bodies in such a manner that overspraying is very likely to occur. In addition, the air spraying devices have been made very complicated with a multitude of flow passages and the like.
It would be desirable to thave an air-type spraying device for air spraying coatings on the internal surfaces of longitudinal seams of can bodies in such a manner that overspraying and splattering are substantially eliminated and the coating is of a substantially optimum thickness.